The application of internal combustion engines, whether stationary or mobile, often requires significant noise, vibration and harshness (“NVH”) engineering to reduce naturally generated sound frequencies. Rotating devices installed in, or associated with, internal combustion engines are a common contributor to such noise. Rotating parts such as fan blades or supercharger lobes may generate sound that varies over a range of frequencies; primarily as a function of the rotational velocity of the component. Additionally, rotating components may also produce noise as they pass by stationary objects.
Under-hood and induction system noise associated with an automotive internal combustion engine is a target of significant NVH focus due to the desirability of providing a quiet and comfortable driving experience for the operator of the vehicle. Induction noise produced by the engine depends on the particular engine configuration and may be affected by such factors as the number of cylinders, and the volume and shape of the intake manifold, plenum and intake runners. The application of induction compression through the use of an engine driven supercharger, or an exhaust driven turbocharger, may also contribute substantially to under-hood noise. Other under-hood sound produced by the engine may be contributed by rotating accessory drives, associated accessories and fans for cooling the engine.
Quarter wave tubes produce a sound-canceling wave of a frequency that is tuned to a wavelength four times longer than the quarter wave tube. Quarter wave tubes are often used to reduce sound generated by engine induction systems, but are typically of a fixed length and are therefore limited to addressing specific frequencies. Noise of varying frequency or noise of several different orders, such as may be produced by variable-speed rotating components, may require the use of multiple quarter wave tubes or other sound attenuation solutions that can be costly, difficult to package and of limited effectiveness.
Accordingly, it is desirable to provide a sound attenuator such as a quarter wave tube that can attenuate varying sound frequencies that are generated by rotating devices.